[62.03] The Origin of the UV Upturn in Elliptical Galaxies

S. Yi (NASA/GSFC)

The unexpectedly high ultraviolet flux in the spectra of
giant elliptical galaxies, known as the UV upturn, has been
one of the most controversial topics ever since space
observations with UV capability became available.
This thesis presents a population synthesis and analysis
which concludes that this phenomenon is a natural result of
the advanced stellar evolution of a metal-rich population of
stars. All three important empirical discoveries about the
UV upturn - the presence of a UV upturn only in giant
elliptical galaxies, the positive UV upturn-metallicity
relationship, and the narrow range of the T_eff of
UV sources - are precisely reproduced.
The achievements are as follows:

The origin of the UV bright phase of metal-rich core
helium burning stars has been understood.
More metal-rich core helium-burning stars become UV
bright more easily when
Z \stackrel\textstyle>\raisebox-0.7ex\sim\, Z_ødot
because their
lifetime is longer due to their smaller mass and their hot
helium-burning core gets exposed earlier as the hydrogen
shell burns up more quickly.
I provide an analytic derivation of the origin.

(2) This first detailed sensitivity study of the resulting
UV spectrum to various input parameters shows that the UV
flux is very sensitive not only to age and metallicity, but
also to the treatments of the mass loss efficiency
and the HB mass distribution.

(3) The origin of the UV upturn has been uncovered.
Under the conventional assumptions, only old, metal-rich
model galaxies develop a sufficient number of UV bright HB
and evolved HB stars to generate UV upturns of the observed
properties introduced above within a Hubble time.
Accordingly, important clues to the evolution of giant
elliptical galaxies are discussed.